Refining of lead



June 24, 1947. w. T. ISBELL 2,422,967

REFINING 0F LEAD Filed Nov. 20,- 1944 72 Mum/m Him o Patented June 24, 1 7

DEFINING LEAD e William T. Isbeil, Hercnianeum, Ma, aaignor to St. Joseph Lead Company, New York, N, Y., a

corporation of New York Original application May 25, 1944, Serial No. 537,265. Divided and this application November 20, 1944, Serial No. 564,320

Claims. (01. 286-37) The present invention relates to apparatus for the removal of zinc from lead, particularly from lead which has been desilverized by treatment with zinc. 4

In the refining of lead, it is common practice to remove silver from the lead by the addition of zinc which forms with the silver a zinc-silver compound substantially insoluble in lead saturated with zinc at temperatures of the order of 900 F. However, after removal of the zincsilver crusts the lead contains from 0.5 to 0.6%

of zinc which must be removed for most of the commercial uses of lead.

Until recently it hasbeen the usual practice to remove the remaining zinc from the lead by heating the lead to red heat in an oxidizing atmosphere, whereby the major portion of the zinc together with a substantial portion of lead were oxidized to form a slag or dross which was skimmed off and resmelted to recover the lead content. 2

It was proposed at an early date to remove the zinc by distillation but this method was found to require a relatively high temperature and a large proportion of the lead volatilized with the zinc at the temperature of distillation.

More recently it has been proposed to remove zinc from lead by treating the molten lead with oxidizing agents such as chlorine or nitrates, or with caustic'soda, or with mixtures of caustic soda and nitrates. All of these methods require the use of expensive chemical reagents and the zinc is converted into salts or oxide of reduced value.

I have now found that the disadvantages of the previously proposed methods may be avoided and to the vacuum. An advantageous apparatus for practicing the new lead reflningprocess comprises a container for the lead to be refined, means for heating the container, a bell member open at the lower end, preferably at least about forty inches in depth, and adapted to be immersed in a charge of molten lead in the container, means for exhausting theinterior of the bell member and means for cooling the upper surface of the bell member. A suitable form of 1 apparatus is shown by wayof example in the eeflat top surface of the bell is provided with a.

the zinc content of the lead removed and recovered in the form of metallic zinc with substantially no loss of lead and without the use of either high temperatures or chemical reagents, by subjecting molten lead containing zinc to low pressures. It has been found that at absolute pressures not exceeding about 10 mm. and preferably from 1 mm. to 0.5 mm. of mercury and at temperatures of from 900 F. to l200 F., preferably at about 1080 F. to 1125 F., the zinc content of the lead volatilizes readily with a relatively small proportion of lead and may be condensed and recovered in metallic form.

I have found it to be particularly desirable to condense the zinc vapors upon a cooled surface positioned relatively close to the free surface of the molten lead under vacuum and to provide means. such as a suitable agitator, for continually circulating the molten leadto the surface exposed companying drawing.

In the drawing I is a kettle of, for example, 120 tons capacity mounted in a furnace setting 2 for maintaining a charge of zinc-containing lead in molten condition. 3 is a removable vaporizer-condenscr comprising a bell 4, in the shape of a truncated inverted hemisphere substantially enclosed except for an, opening in the u bottom thereof at least forty inches below the top surface for access of the molten zinc-containing lead into which the bell is placed. The

water-cooled jacket 5 and a vacuum line 6, leading to a vacuum pump, opens into the interior of the bell just below the top surface. As will be seen from the drawing, the flat top surface of the bell extends in proximity to the surface of the charge when under the efiect of the vacuum. A stirring propeller I mounted in agitator well 8 is driven by motor 9. The propeller shaft is positioned within a tube l3 extending through the top of the bell to at least the level of the lower opening of the bell.

The vaporizer-condenser assembly is carried on a framework I 0 adapted to be mounted on studs ll carried by the furnace setting 2.

The following is a typicalexample of the operation of the apparatus of the invention.

A charge of about tons of lead containing about 0.5% of zinc is placed in kettle I with the vaporizer-condenser removed, and heated to 1100 F. The lead is skimmed free of dross, and the vaporizer-condenser is put in position as shown in the drawing, and water connections to the cooling jacket are made. The temperature of the lead normally drops about 50 F. when the vaporizer-condenser assembly is submerged in the charge. When the temperature has been brought back to 1090 F., the stirrer and the-vacuum pump are put in operation. The temperature is preferably maintained at about 1090 to 1100 F. and the pressure at about 0.5 mm. of mercury during the dezincing treatment which for a batch of this size requires about seven hours,

' stirrer and vacuum pump are stopped, the water In general, about 90% of the zinc content of 1 the lead is recovered, and the lead after treatment contains 0.05% or less of zinc. The recovered metallic zinc is useful in the desilverizing of further portions of lead.

Any cadmium which may 'be present in the lead, for example, from the ore or'from the zinc used in the desilverizing process, will be eliminated from the lead by the treatment of the invention. It is therefore possible, when using the treatment of the invention, to use zinc having a substantial cadmium content in the desilverizin of lead.

Although the apparatus shown and described for the purpose of illustrating the principles of the invention has been found to be particularly useful, the process of the invention is not limited to any particular form of apparatus but may be practiced in any apparatus capable of subjecting the lead to a vacuum ofthe order of 10 mm. of

mercury or less at a temperature of from 900 F. to 1200 F.

ing means for maintaining the lead in the container in molten condition, a bell member adapted to fit into 'said container, constituting a substantially closed chamber except for an opening at the bottom for the access of said molten lead, means for evacuating the interior of the bell member, means for constantly renewing the surface of the lead charge in said container comprising an impeller and guide means surrounding said impellerto direct flow of molten charge .towards the surface of the charge, and means for cooling the upper portion of the bell member.

4. Apparatus for the vacuum evaporation of zinc from zinc-bearing lead comprising a container adapted to hold a charge of lead, means for maintaining the lead in the container in molten condition, a bell member adapted to fit into said container constituting a substantially closed chamber except for an opening at the bottom for access of said molten lead, means for evacuating the interior of the bell member, agi-.

tator means for constantly renewing the surface of the lead charge in said container, said bell member providing a flat top portion extending in proximity to the surface of the lead charge under a vacuum of the order of 10 mm. of mercury, and means for cooling said flat top a portion of the bell member.

This application is a division of my applica+ tion Serial No. 537,265, filed May 25, 1944. I claim: 1. Apparatus for the vacuum evaporation of zinc from zinc-bearing lead comprising a container adapted to hold a charge of lead, heating means for maintaining the lead in the container in moltencondition, a bell member adapted to fit into said container constituting a substantially closed chamber except for an opening at the bottom for the access of said molten lead, means for evacuating the interior of the bell member, and means for cooling the upper portion of the bell member. 1

2. Apparatus for the vacuum evaporation of zinc from zinc-bearing lead comprising a container adapted to hold a charge of lead, heating means for maintaining the lead in the container in molten condition, a bell member at least about forty inches in depth adapted to fit into said container, constituting a substantially closed chamber except for an opening at the bottom for the 'acess of said molten lea'd, means for evacuating the interior of the bell member, and means for cooling the upper portion of the bell member.

3. Apparatus for the vacuum evaporation of zinc from zinc-bearing lead comprising a container adapted to hold a charge of lead, heat- 5. Apparatus for the vacuum evaporation of zinc from zinc-bearing lead comprising a container adapted to hold a charge of lead, means for maintaining the lead in the container in molten condition, a bell member adapted to fit into said container constituting a substantially closed chamber except for an opening at the bottom for access of said molten lead, means for evacuating the interior of the bell member,

agitator means for constantly renewing the surface ofthe lead charge in said container, in-

' cluding a tube open to the atmosphere above the top of said bell member and extending vertically at least to the lower level of said bell 'member, an agitator shaft extending through said tube, an agitator carried by said shaft below the lower end of said tube,-and drive means connected to said shaft above the upper end of said tube.

. WILLIAM T. ISBELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS I 

